Low-substituted hydroxypropyl cellulose is cellulose ether which is insoluble but swells in water owing to a low degree of substitution. Low-substituted hydroxypropyl cellulose is industrially produced by reacting cellulose in powder, chip, or sheet form with propylene oxide in the presence of an alkali, followed by dissolving, neutralizing, and washing the reaction product.
The washing step is usually performed successively by using a continuous horizontal vacuum filter, a horizontal table filter, or a horizontal belt filter. After washing, the filtered low-substituted hydroxypropyl cellulose is subjected to compression type dehydration in order to reduce the burden of a dryer and reduce the amount of water-soluble salts formed as a by-product.
JP 08-231602A describes a method for dehydrating water-soluble cellulose ether by using mainly a V-shaped disc press, which is a compression type dehydrator. According to JP 08-231602A, this method makes it possible to reduce the water content of the dehydrated water-soluble cellulose ether even to 35 to 40% by weight.